(a) Field of the Invention
The present invention relates generally to switching power supplies and more particularly to digitally controlled voltage converters.
(b) Related Art
In low power applications such as portable electronic devices, a buck inverter or boost converter, is generally utilized depending on the voltage of the power source and the voltage needed to power the circuitry of the portable electronic device.
For example, a portable electronic device may be designed to be powered from a lithium battery that produces a supply anywhere between 4.2 volts and 3.0 volts, while CMOS integrated circuits in the device require a supply of 1.8 volts to 2.5 volts. In this example, a buck converter would be utilized to step down the battery voltage to a controlled 1.8 or 2.5 volts. If, however, the same portable electronic device were designed to be powered from a 1.5 volt battery, the device would include a boost converter to step up the 1.5 volts to 1.8 or 2.5 volts.
Switching voltage converters, alternately store and output energy. Energy from the input voltage is temporarily stored in an inductor during each switching cycle. Generally, a switch driven by a series of pulses controls how much of the energy is output. A filter is used to smooth the output into a DC voltage. The output DC voltage can be higher or lower than the input voltages. The output DC voltage may also be negative with respect to the input voltage.
Switching voltage converters operate in either a discontinuous mode or a continuous mode. In the discontinuous mode, controllers completely de-energize the inductor before the end of every switching cycle. Thus, there is no current in the inductor at the start of every switching cycle in the discontinuous mode. In the continuous mode, controllers do not completely de-energize the inductor before the end of every switching cycle. Thus, the current in the inductor never reaches a point where there is no current in the inductor in the continuous mode.
The output of a switching regulator is determined in part by the duty (cycle) ratio. The duty ratio is equal to the time period in which the switch is “on” divided by the time period of the switching cycle (D=Ton/T). The switching cycle time period is equal to the time period in which the switch is “on” plus the time period in which the switch is “off” (T=Ton+Toff).
There are four basic topologies of switching regulators: the buck converter, the boost converter, the non-inverting buck-boost converter, and the inverting buck-boost converter or inverter. The output voltage of the buck converter is equal to the input voltage multiplied by the duty ratio (Vout=Vin*D). Thus, the output voltage of the buck inverter cannot be greater than the input voltage. Also, the output voltage of the buck converter cannot be negative.
In the boost converter (or “step-up” converter), the output voltage is equal to the input voltage times the switching cycle time period divided by the time period in which the switch is off (Vout=Vin*T/Toff). Thus, the output voltage of the boost converter cannot be less than the input voltage. Also, the output voltage of the boost converter cannot be negative with respect to the input voltage.
For the buck-boost and single ended primary inductor converter (“SEPIC”), the output voltage is equal to the input voltage multiplied by the ratio of the time period in which the switch is on to the time period in which the switch is off (Vout=−Vin*Ton/Toff). Thus, the output voltage has a magnitude that is either stepped-up or stepped-down from the input voltage as determined by the duty ratio.